Mechanical parts designed for use in severe environments are often subject to extreme mechanical stress. In critical environments, such as in aircraft engines, these parts must have a degree of reliability built into them that allows them to operate with the lowest possibility of failure. Often such mechanical parts are subject to failure due to the stress at particularly highly stressed locations within the parts. Anything that tends to concentrate stress in one location or another can give rise to failure.
In particular, sharp breaks, such as edges or corners formed when a tool cuts through a piece of material, sharp bends in a material, or surface irregularities can all cause stress to concentrate causing susceptibility to failure. Edges created in the machining of parts can act as concentrators of stress if the edges are not properly finished. Edges are particularly vulnerable to undesirable stress concentration when the edge occurs in an area where high stress is already present.
One way to minimize stress concentrators, also known as stress risers, is to carefully finish surfaces eliminating burrs and scratches. In the example of an edge, finishing the at risk edge very smoothly and replacing the sharp break of the edge with a radiused edge, multi-axis machined edge, or other edge treatment having known and specifically chosen characteristics can serve to retard the concentration of stress. Once a surface has been appropriately treated, it is critical to measure the results of the treatment and to critique the treated surface for adherence to engineering specifications.
Historically, edges have been measured by a variety of methods. Prominent among these is a method called wax-and-trace. This method relies on an operator to apply an impressionable material, such as a heated wax, to the desired part feature by hand, allow the wax to cool, and remove the wax, all the time positioning and holding the wax by hand. All this is to be done without distorting the profile of the impression. The removed impression in the wax is then visually aligned in a profiling machine often using a clay mount and trying to ensure that the direction at which the tracing element of the profiling machine approaches the impression is normal to contour centerlines. In addition to contact or tracing methods there are numerous other methods for profiling an impression among which are optical methods, interferometric methods, acoustic methods, or other methods that may be used in profiling.
The above procedure can lead to some inaccuracies. Shortfalls include: inability to reliably locate the feature to be measured; impressionable material deformation during application, cooling, and removal; and inability to accurately align the impressionable material to the profiling machine sensors.
Accordingly, a need exists to be able to make impressions of the features of surfaces that do not suffer from the limitations of holding the impressionable material by hand and that allow repeatable and accurate placement in a profiling machine for accurate measurement of the features of surfaces.